Many types of modern hydraulic machines, including forestry machines, excavators, and material handlers, will often lift large loads and then at some later time lower them in a controlled fashion. To perform this lowering, hydraulic fluid, under pressure, will be allowed to leave the hydraulic cylinder(s) holding the load in an elevated position. This fluid will be passed through an orifice, such as a control valve, and the hydraulic pressure will be converted into heat. Thus, much of the original energy used to elevate the load is lost.
These machines will typically be performing other hydraulic functions, such as boom rotation or machine travel, at the same time as the load is lowering. This will be accomplished by generating hydraulic fluid pressure by using a prime mover, such as a diesel or gasoline engine, to propel a hydraulic pump. Thus the machine is regularly converting stored pressure into heat, and at the same time creating new pressurized fluid to perform these other hydraulic functions.